Silicon carbide foam with high specific surface area and improved mechanical properties

ABSTRACT

Silicon carbide foam useful as a catalyst support has a BET specific surface area of at least 5 m 2 /g, and a compression strength exceeding 0.2 MPa. The foam is prepared by impreganting an organic foam with a suspension of silicon in a resin containing a cross-linking agent, incompletely cross-linking the resin, carbonizing the foam and resin, and carburizing the silicon.

FIELD OF THE INVENTION

The invention relates to a silicon carbide foam with high specificsurface area and high porosity, and improved mechanical properties(particularly compression strength), this foam being used mainly as acatalyst support, for example in the chemical or petrochemical industryand in exhaust silencers for internal combustion motors, or filters.

It also relates to its manufacturing process and its applications.

DESCRIPTION OF RELATED ART

Patent FR 2657603 describes how to obtain catalyst supports,particularly made of SiC, with a high specific area (greater than 15m²/g) with a dual mode porosity in which a first pore family with anaverage diameter of between 1 and 100 μm enables a gas to access asecond pore family with average diameters smaller than 0.1 μmresponsible for the specific area and the catalytic activity.

This support is obtained by mixing an Si powder or one of its reduciblecompounds in a polymeric or polymerizable organic resin, possibly withadditives, forming of the mixture, cross-linking and polymerization ofthe resin, obtaining a porous carbon skeleton containing Si or acompound of Si, by carbonation in a non-oxidizing atmosphere at atemperature of between 500 and 1000° C., and finally carbonation of Siat a temperature of between 1000 and 1400° C., still under anon-oxidizing atmosphere.

This type of support has good compression strength and a fairly highdensity, usually of the order of 0.6 to 0.8 g/cm³, but it looks morelike a solid porous body rather than having the normally aeratedappearance of a foam; consequently, its permeability is not sufficientto process large volumes of gas per unit weight of support, and itsrange of applications is limited. In other words, if the dimensions ofthe support are large it becomes difficult for treated gasses to reachits center which is consequently unused dead mass.

Patent FR 2684092 describes an SiC foam obtained by a carburizationreaction starting from a volatile compound of Si with an activatedcarbon foam. This activated carbon foam may result from a polyurethanefoam reinforced by impregnation using a resin, setting up the resin,carbonation and activation.

The carbide foam obtained has a specific area of not less than 20 m²/gdue to macrbpores containing edges with lengths varying from 50 to 500μm, and mainly mesopores for which the diameter is usually between 0.03and 0.05 μm, which is generally about three times larger than thediameter of the pores in the activated carbon foam.

Its density is between 0.03 and 0.1 g/cm³, however its relatively modestmechanical strength (the compression strength does not exceed about 0.02MPa) may limit its field of use or may necessitate specific treatmentsto strengthen it if necessary.

Patent FR 2705340 describes a process for making a silicon carbide foamthat consists of starting from a polyurethane foam, impregnating it witha silicon suspension in an oxygenated organic resin (usuallyfurfurylic), polymerizing the resin up to 250° C. at a rate of 5°C./min., simultaneously carbonizing the foam and the resin between 250and 1000° C. under an inert atmosphere, carbonizing the Si contained inthe resulting carbon foam up to a temperature of between 1300 and 1600°C., and maintaining this temperature for 2 h under an inert atmosphereand cooling the carbide obtained.

The carbide foam obtained has a specific area of not less than 5 m²/g,which in particular depends on the maximum temperature reached. It has atwo mode porosity comprising macropores with an average diameter ofbetween 100 and 150 μm and mesopores between 0.0275 and 0.035 μm.

This foam may be used as a catalyst support or as a diesel enginefilter.

It gives satisfactory results in catalytic reactions. However as before,its compression strength and its resistance to abrasion are insufficientwhen severe thermal and/or mechanical loads are applied to it,particularly for use in exhaust silencers.

Thus the petitioner has attempted to make the use of the said SiC foamsupports more reliable, particularly in exhaust silencers or forregeneration treatment, by significantly improving their mechanicalproperties without penalizing their catalytic properties, particularlytheir specific area or dual mode porosity (which is not easy sinceusually one is obtained at the detriment of the other) while maintainingtheir permeability.

Therefore, the petitioner attempted to improve the foam skeleton.

SUMMARY OF THE INVENTION

The invention is a foam based on silicon carbide for catalyticapplications with a high specific area (its BET area is typically atleast 5 m²/g), characterized in that it has a compression strengthhigher than 0.2 MPa (2 bars), but generally at least 0.4 MPa (4 bars).

DETAILED DESCRIPTION OF THE INVENTION

The foam according to the invention usually has a bimodal porosity,measured with mercury, comprising essentially a family of pores with anaverage diameter of between 10 and 200 μm enabling easy access of thegasses to be processed towards mesoporosity in which the averagediameter of the pores is between 0.005 and 1 μm, and which enables thecatalytic activity.

This bimodal porosity is additional to the porous structure of the foamwhich is typically in the form of a network that could be qualified as“fibrous” comprising sorts of communicating cages delimited by carbideedges (or bridges) usually with a thickness of between 50 and 500 μm,connected to each other by nodes. The megapores of this network, visibleto the naked eye, have dimensions that may be between 0.4 and 1.6 mm andcorrespond to a porous volume of about 3 to 12 cm³/g. Consequently, itsnon darcian permeability to air is at least 10⁻⁵ m at 20° C. Thispermeability measures the ease with which gasses to be catalyticallytreated can pass through it.

It is remarkable to note that the specific area of the foam usuallyexceeds 10 m²/g.

Its density is typically between 0.06 and 0.2 and preferably between0.08 and 0.15.

It is beneficially in the form of a monolithic part, but it may also beused in particular form, in other words as stacked pieces of foam.

The compression strength is measured by a hardness test, well known inthe strength of materials field. It consists of applying a force to acylindrical punch with a known plane section and measuring the forcenecessary to make it penetrate into the foam by a distance of 1 cm, thesample having at least two plane parallel surfaces at a separatingdistance of at least 5 cm.

The foam according to the invention also has very good resistance tothermal shocks.

Thus, it resists at least one thermal shock consisting of increasing itto at least 800° C. and then suddenly cooling it in air to ambienttemperature, without reducing the compression strength.

But it is even more remarkable to note that it resists a succession ofseveral thermal shock cycles, each cycle including heating to hightemperature followed by sudden cooling in air. For example, it wassubjected to a sequence of heating and cooling cycles carried out attemperatures varying from 800° C. to 950° C. at 25° C. intervals, twocycles being carried out at each temperature, without noting anysignificant reduction in its compression strength.

Sudden cooling in thermal shocks takes place at an average rate of about60° C./min.

The SiC content of the foam is typically greater than 95%, or better98%, the residual Si content generally not exceeding 0.1%. The residualC content does not exceed 3%, and normally does not exceed 2%; theresidual C content can be eliminated by oxidation in air at atemperature of about 600° C. to 850° C.

This foam is obtained by impregnating an initial organic foam, usuallypolyurethane, using a suspension of a silicon powder in a resin; thisresin contains oxygen with a carbon yield exceeding 30%, and across-linking catalyst is added to it in the proportion of 1 to 10% (byweight), and preferably 5%; it is usually furfurylic resin and thecross-linking agent is hexamethylenetetramine, the proportion of siliconto resin by weight being between 0.6 and 1.2. The ratio of the totalmass of impregnated foam to the initial foam mass is greater than 10 andless than 20, which usually corresponds to a ratio of the weight ofresin to the weight of foam greater than 5 but not more than 11 to avoidthe risk of blocking the porous structure of the foam. The impregnatedfoam is thermally treated until the resin is incompletely cross-linkedwhen the organic foam is deteriorated, and the organic foam and theresin are then carbonized by increasing the temperature to 1200° C.under an inert atmosphere; the silicon is carbonized, still under aninert atmosphere, by increasing the temperature to between 1200° C. and1370° C. to obtain a carbide foam with a high specific area, or to ahigher temperature when obtaining a very high specific area is lesscritical, for example when the carbide foam is used as a filter in adiesel engine.

As already mentioned, the initial organic foam is usually a shaped part.Beneficially, it may comprise a doping element that improves theresistance of the SiC foam to oxidation at high temperatures, forexample a powder of at least one easily oxidizable metal such as Al, Ca,Y, etc. or an alloy containing these metals, this doping element beingintroduced into the foam mass, for example during its manufacture.Furthermore, it is unexpectedly observed that addition of these dopingagents usually improves the mechanical properties of the final carbidefoam, particularly its compression strength.

If necessary, the permeability of the said organic foam can be improvedby preliminary treatment, for example with soda in the case ofpolyurethane.

Instead of starting from an organic foam (usually polymeric and possiblydoped), the invention also includes the possibility of starting fromcomponents used to make foam (for example monomer or copolymerizableagents, porogenic additives, hardeners, cross-linking or other agents)to which the said doping agent may be added, and optionally adds thesilicon suspension in the resin to this mixture. This mixture may thenbe formed by molding, injection, etc. before obtaining the foam andbeing heat treated.

The Si suspension in an organic resin may contain several additives;solvent (for example alcohol) with filler (for example carbon black) toadjust the viscosity, plastifying agent, surface tension agent, etc. Inthis case, a heating step may be carried out at a moderate temperatureto eliminate solvents, keeping the temperatures within the thermalconditions mentioned above.

The grain size of the silicon powder usually passes the 50 μm sieve, andpreferably has an average particle diameter of less than 10 μm; it maybe introduced in the form of an alloy comprising the said dopingelements in order to improve the resistance of the SiC foam tooxidation; these doping agents may also be added in the form of ametallic powder or in the form of a decomposable salt mixed with thesaid Si powder. The proportion of the doping elements typically does notexceed 10% of the silicon added into the resin.

The polymerized resin typically contains not less than 5% by weight ofoxygen, and preferably 15%.

It is essential to perform incomplete cross-linking of the resin beforethe organic foam deteriorates. The remaining plasticity accommodates anydimensional variations, deformations and stresses that occur duringtransformation of the said carbon foam during subsequent heattreatments. Thus, the risks of defects in the foam skeleton, resultingin the presence of voids in bridges, bridge bonding faults, etc., aresignificantly reduced and improve the mechanical properties. Similarly,the lack of constraints makes a significant contribution to improvingthe strength of the carbide foam, and particularly its resistance tothermal shock.

The incomplete polymerization ratio may be characterized by measuringthe vitreous transition temperature (Tg) of the partially polymerizedresin. In general this temperature is less than 110° C. and correspondsto the appropriate degree of polymerization when starting carbonation;and it is greater than 70° C., otherwise the shaped parts will not havesufficient resistance during the heat treatment.

The controlled polymerization heat treatment may be carried out indifferent ways; it is usually adapted to the size of the treated parts.

For example, the part could be heated by oven heating at a temperatureof less than 225° C., typically between 150 and 225° C. and preferablyabout 200° C., for a period of between 10 and 90 minutes (preferablybetween 60 and 90 minutes) and then possibly cooled before continuingthe heat treatment. It is also possible to work more quickly at a highertemperature, by putting the part in an oven preheated to a temperatureexceeding the temperature at which the organic foam is degraded, forexample 300° C., and limiting the residence time in the oven so thatdegradation of the said organic foam takes place before completepolymerization of the resin.

The polymerized resin typically contains at least 5% (by weight) ofoxygen, and preferably 15%.

Note also that in combination with the controlled polymerization heattreatment, the high proportion of resin and therefore the impregnationsuspension added to the organic foam contributes to increase themechanical properties, and particularly the compression strength,without affecting the specific surface area which characterizes thecatalytic properties of the carbide foam.

This type of carbide foam with a good compression strength could be usedas a catalyst support in divided form of stacked pieces; but it isparticularly suitable for use in monolithic form, for example in exhaustsilencers; all that is necessary is to cover it with a deposit of therequired catalyst, using conventional processes.

These mechanical properties of the foam according to the invention alsomake it particularly suitable for treatment after use in order torecover the deposit of catalyst covering it using simplehydrometallurgical processes and/or in order to recycle it.

The process may also be complemented by a stabilization heat treatmentstep in an oxidizing atmosphere, in order to improve the resistance ofthe silicon carbide foam to oxidation. This treatment may be done duringelimination of the residual carbon; it is particularly beneficial to dothis when the foam contains a doping element. It is typically carriedout at between 850 and 1200° C. for a period of between 5 minutes and 24hours, or preferably between 950 and 1100° C. for between 15 minutes and10 hours, the duration being longer when the temperature is lower. Itresults in the foam being coated with an oxide film containing oxides ofat least silicon or the doping elements, silicon oxide usuallycontaining oxides of doping elements.

In order to give good resistance to oxidation, it is also possible toimpregnate foam (for example under vacuum) using a solution of adecomposable salt of at least one of the said doping agents, applying aheat treatment to decompose the salt, and it is then advantageous tofinish the treatment by applying the previous stabilizing treatment toobtain the corresponding protective film.

The following examples illustrate this invention.

EXAMPLE 1

This example concerns a silicon carbide foam obtained according to aprocess based on the state of the art.

This process is of the type described in patent FR 2705340.

A cylindrical piece of polyurethane foam with a diameter of 14 cm andheight of 8 cm with a density of 0.028 was impregnated using asuspension containing Si powder with an average grain diameter of 5 μmin 95% furfurylic alcohol and 5% hexamethylenetetramine acting as apolycondensation catalyst.

The ratio of the silicon mass to the resin mass is 0.7.

After impregnation of the polyurethane foam by the suspension, the ratioof the weight of resin to the weight of the said foam is 4.1 and theratio of the total mass of impregnated foam to the mass of polyurethaneis 7.8.

Polymerization took place by increasing the temperature up to 250° C. ata rate of 5° C./min. for 45 minutes and holding the temperature constantat 250° C. for 5 minutes in order to polymerize the resin.

The vitreous transition temperature (Tg) of this resin under theseconditions is 118° C.

Carbonation was then done by increasing the temperature from 250° C. to1000° C. under an Ar atmosphere at a rate of 1° C./min.

The heat treatment continued by increasing the temperature up to 1350°C. at a rate of 3° C./min. with a constant temperature of 2 h at 1350°C., always under an inert atmosphere.

The resulting carbide foam was then treated in air at 800° C. to destroythe residual carbon.

The BET specific area is then 10.8 m²/g and the compression strengthmeasured by the hardness test is 0.08 MPa.

EXAMPLE 2

This example illustrates the invention.

The starting point was a piece of polyurethane foam identical to that inexample 1.

The foam impregnation suspension was made using a silicon powder with anaverage grain diameter of 5 μm, in furfurylic alcohol with 5% ofcross-linking catalyst (hexamethylenetetramine).

The ratio of the mass of Si to the mass of resin is 0.7.

However, the ratio of the mass of impregnated foam to the mass ofpolyurethane is 16.

Incomplete polymerization was done by over drying, increasing theimpregnated foam temperature to 200° C. with a rate of temperature riseequal to 5° C./min.

The time did not exceed 35 minutes.

The value of Tg is 103° C.

The hardened product was then placed in a furnace under an Ar atmospherein which the temperature was increased to 1200° C. at a rate of 3°C./min., to perform carbonation.

The heat treatment was continued by increasing the temperature up to1350° C. under the same conditions, with the final temperature beingheld constant for 2 h to carbonize the silicon.

The shaped part of Si carbide foam has a BET specific area of 11.2 m²/gand a compression strength of 0.6 MPa, which makes it particularlysuitable for being impregnated by a catalyst for use in an exhaustsilencer.

EXAMPLE 3

This example illustrates how to obtain a carbide foam with a dopingagent according to the invention.

The initial polyurethane foam is impregnated using the same suspensionas in example 2, containing Si in furfurylic alcohol with across-linking catalyst; however, monohydrated aluminum nitrate was addedin a quantity sufficient to give 0.75% (by weight) of Al as a percentageof the final weight of SiC.

Heat treatments are the same as in example 2.

The SiC foam obtained has a specific area of 11.7 m²/g, which is of thesame order of magnitude as that in example 2; however, the compressionstrength of 0.9 MPa is significantly higher.

The SiC foam part was separated into two pieces. One of them wassubjected to a stabilization treatment at 1000° C. for 2 hours in air;however, the two were then submitted to an oxidation resistance test byexposure to air at 1100° C. for 5 hours.

The weight increase of the unstabilized part was 9.3%, whereas for thestabilized part it was 1.6%.

For comparison, the same undoped foam (example 2) had a weight increaseof 15.8% under the same conditions when it was not stabilized, and 6.7%when it was stabilized.

What is claimed is:
 1. Silicon carbide foam for catalytic applicationswith a BET specific surface area of at least 5 m²/g, and having acompression strength exceeding 0.2 MPa.
 2. Foam according to claim 1,wherein the BET specific surface area is at least 10 m²/g.
 3. Foamaccording to of claim 1, having a bimodal porosity additional to theporous structure of the foam, comprising a first family of macroporeswith an average diameter of between 10 and 200 μm and a second family ofmesopores with an average diameter of between 0.005 and 1 μm.
 4. Foamaccording to claim 1, wherein the compression strength is maintainedafter being subjected to a thermal shock at not less than 800° C. 5.Foam according to claim 1, additionally comprising at least one dopingelement.
 6. Foam according to claim 5, wherein the at least one dopingelement is an oxidizable metal.
 7. Foam according to claim 6, whereinthe oxidizable metal is Al, Ca or Y.
 8. Foam according to claim 1,additionally comprising a coating with an oxide layer to improveresistance to oxidation of the foam.
 9. Foam according to claim 8,wherein the oxide layer comprises at least silicon oxide or a dopingelement oxide.
 10. Process for making a silicon carbide foam forcatalytic applications with a BET specific surface area of at least 5m²/g, and having a compression strength exceeding 0.2 MPa, comprisingthe steps of: impregnating an organic foam of predetermined permeabilitywith a silicon powder suspension in a polymerizable resin containing across-linking catalyst, the resin containing oxygen and having a carbonyield of at least 30%, with a weight ratio of impregnated foam to foambefore impregnation being between 10 and 20, treating the impregnatedfoam to cause cross-linking followed by degradation, the resin beingincompletely cross-linked when degradation begins, carbonizing theorganic foam and the incompletely cross-linked resin simultaneously byheating to 1200° C. under an inert atmosphere, and carburizing thesilicon by heating to at least 1370° C., thereby obtaining siliconcarbide foam with a residual Si content of less than 0.1%.
 11. Processaccording to claim 10, wherein the organic foam is a polyurethane foam.12. Process according to claim 10, wherein the organic foam contains atleast one doping element.
 13. Process according to claim 10, wherein theincompletely cross-linked resin contains at least 5% oxygen by weightand a carbon yield of at least 30%.
 14. Process according to claim 13,wherein the resin is a furfurylic resin.
 15. Process according to claim10, wherein the resin contains at least one doping agent.
 16. Processaccording to claim 10, wherein incomplete cross-linking is performedsuch that the resin has a vitreous transition temperature of not morethan 110° C. when carbonization begins.
 17. Process according to claim10, wherein the silicon powder has a size of less than 50 μm. 18.Process according to claim 10, wherein said silicon is added in the formof an alloy containing at least one doping element.
 19. Processaccording to claim 10, additionally comprising a stabilization treatmentstep of heating the silicon carbide foam in an oxidizing atmosphere at atemperature of between 850 and 1200° C. for a period of between 5minutes and 24 hours.
 20. Process according to claim 19, wherein thestabilization treatment takes place at a temperature between 950 and1100° C. for a period of 15 minutes to 10 hours.